1. Field of the Invention
The present invention relates to the field of tightness testing and, more particularly, to a method of and an apparatus for testing the tightness of two valves arranged upstream respectively downstream of each other in a fluid line by measuring the pressure in the line portion between said two valves with said two valves being closed.
2. Prior Art
Two valves arranged in series are fitted for safety reasons mainly in lines taking fuel gas to gas-burning apparatuses. The tightnesses of said valves are tested at least prior to the start-up of such a gas-fuelled apparatus and in many cases after the shut-down of such an apparatus to establish when gas first leaks across one of said valves at a rate in excess of a predetermined letby rate. The cost of such a tightness test must be reasonably low, since it would otherwise be more cost-effective to incorporate a third valve in such a fuel gas line and to shorten maintenance intervals.
One of the conventional methods of testing the tightnesses of two valves arranged in such a manner in a fuel gas line provides for venting the line portion between said two valves and for measuring the pressure in said line portion after it has been vented. If no pressure build-up in said line in excess of a defined maximum pressure build-up is then measured, the upstream valve is judged to be substantially tight. Said line portion is thereupon filled and the pressure therein is again measured. If no pressure decay in excess of a maximum pressure drop is then found during said second measurement, the downstream valve is judged to be sufficiently tight. The conventional method hereinbefore described hence employs a pre-determined program implying always two switching operations each followed by a pre-determined measuring tim interval, making said method somewhat inflexible and relatively time-consuming.
The West German patent application No. 3 409 050 proposes the use of a pump to increase the pressure in the line portion between two such valves to a pressure in excess of the pressure upstream of the upstream valve and claims that if the pressure in said line portion does not rise to a pre-determined level within a pre-determined time period, pressure measurement will show which one of said two valves is not sufficiently tight. The method proposed by said patent application does not reflect actual operating conditions, though, because the pressure build up in the line portion between said two valves reduces the load to which the upstream valve is exposed under operating conditions and increases the load to which the downstream valve is exposed under operating conditions. The method divulged by the West German patent application No. 3 409 050 is also relatively costly, since it requires the use of a pump operating at a flow rate which must be in keeping with the valve size.
The West German patent application No. 3 445 281 finally proposes a pre-determined pressure below the pressure upstream of the upstream valve to be obtained in the line portion between the two valves for the test. Said patent application continues to say that said relatively low pressure in said line portion must be monitored to establish whether said pressure increases or decreases at an unacceptable rate, the upstream valve not being sufficiently tight in the first case and the downstream valve not being sufficiently tight in the latter case. The reliability of the method proposed by said patent application No. 3 445 281 is inadequate, though, since both valves are taken to be sufficiently tight in the event of a failure preventing the detection or the signalling of a change in the pressure in the line portion between said two valves.